1. Field of the Invention
The present invention relates to a UHF transmitting and/or receiving antenna in the form of a helical antenna for electromagnetic waves in the frequency range of between 400 MHz and 1000 MHz. The helical antenna is arranged within a closed housing which is permeable to HF radiation. The UHF signal is supplied to an end of the helical antenna through a coaxial connector. The helix of the antenna has at least one and a half turns but not more than ten turns. Diameter, height and total length of the stretched-out wire are very small in comparison to the wave length. A mechanically operated device permits a continuous change of the height of the antenna helix along the axis thereof.
2. Description of the Related Art
Helical antennas of the above-described type are known and are described, for example, in the book "Antennas" by John D. Kraus, McGraw Hill Book Company, 1950, chapter 7, pages 173 to 216. When the geometric dimensions of the antenna, primarily the length of the turns, remain small as compared to the wave lengths, the state of radiation of the helical antenna in the distant field is equal to that of a dipole antenna. The direction of maximum radiation of the antenna extends in the distant field in a plane extending perpendicularly to the helix axis, so that the helical antenna operates as an omnidirectional antenna with the axis of the helix as the axis of symmetry. The distant field of such a helical antenna is an elliptic field which becomes a circularly polarized field under the condition ##EQU1## wherein D is the diameter of the helix and s is the pitch of the turns of the helix.
Compared to a .lambda./4 dipole antenna, the helical antenna provides the advantage that it can be of geometrically smaller size for radiating the same wave length without losing substantial transmission power as compared to a rod antenna. For example, the structural height of a helical antenna can be reduced to 20% as compared to a .lambda./4 dipole antenna, while maintaining an efficiency of 80% of the .lambda./4 dipole antenna. Since the helical antenna naturally has a high input resistance, accommodating connections, as they are usually required when the height of dipole antennas is reduced, are not necessary.
However, the helical antenna has the disadvantage that it has only a very small band width, for example, .+-.1.5% of the transmission frequency, which makes it impossible to use the antenna as an individual antenna in a wide frequency band. The attempt to expand the band width of the transmission frequency by tuning with an adjustable series capacity is not very successful because the tuning range is usually not greater than 5% and because the series capacity additionally leads to an accommodation error. In specific cases, another disadvantage is the fact that stray capacitances, such as, a hand or another part of a human body can act near the antenna, and the previously carried out tuning of the antenna becomes ineffective or the antenna is mistuned.
Helical antennas whose lengths are techanically adjustable are well known from U.S. Pat. No. 3,524,193; 3,510,872; 4,475,111; 3,699,585; 3,836,979; and 4,068,238. However, these antennas are exclusively those which are foldable, collapsable or telescoping and in which the reduction or increase of the height is only carried out to be able to better transport them.
A tunable antenna is known from U.S. Pat. No. 4,214,246 in which electric sliding contacts short-circuit one or more turns of an antenna coil in order to tune the antenna. In this case, either the coil itself serves as an antenna or as a tuning element for an antenna rod connected in series with the coil. The primary advantage of such an antenna arrangement is the fact that remote-controlled continuous tuning of the antenna can be carried out.
U.S. Pat. No. 4,169,267 describes a wide-band helical antenna whose optimum antenna gain is to be in the frequency band of from 773 MHz to 1067 MHz. This requirement is met by constructing the helix of the entire antenna from individual sections, wherein the individual cylindrical helically-shaped sections have different lengths and different diameters and are provided with conically extending transition pieces also in the shape of a helix. The specific arrangement of the individual sections makes possible the optimum adjustment to the required frequency band, to the antenna gain, the directional pattern etc. However, an antenna constructed in this manner will require a substantial amount of space which is not to be underestimated.
Another antenna known from U.S. Pat. No. 4,087,820 is intended, for example, for the short wave range between 2 MHz and 32 MHz. The antenna height is, for example, 35 feet, wherein a movable part of the helix permits the continuous change of the height with the pitch remaining constant, so that the antenna can be tuned to resonance in a very wide frequency range. The disadvantage of this antenna is primarily the extremely large height and comparison to the wave length and the fact that it is relatively difficult to transport because the tubular antenna housing which receives the antenna cannot be reduced in size.